1. Field
The disclosure is directed to multimedia signal processing and, more particularly, to techniques for video encoding and decoding channel switch frames (CSF) to enable acquisition and re/synchronization of the video stream while preserving compression efficiency.
2. Background
Multimedia processing systems, such as video encoders, may encode multimedia data using encoding methods based on international standards such as Moving Picture Experts Group (MPEG)-1, -2 and -4 standards, the International Telecommunication Union (ITU)-T H.263 standard, and the ITU-T H.264 standard and its counterpart, ISO/IEC MPEG-4, Part 10, i.e., Advanced Video Coding (AVC), each of which is fully incorporated herein by reference for all purposes. Such encoding methods generally are directed to compressing the multimedia data for transmission and/or storage. Compression can be broadly thought of as the process of removing redundancy from the multimedia data.
A video signal may be described in terms of a sequence of pictures, which include frames (an entire picture), or fields (e.g., an interlaced video stream comprises fields of alternating odd or even lines of a picture). As used herein, the term “frame” refers to a picture, a frame or a field. Video encoding methods compress video signals by using lossless or lossy compression algorithms to compress each frame. Intra-frame coding (also referred to herein as intra-coding) refers to encoding a frame using only that frame. Inter-frame coding (also referred to herein as inter-coding) refers to encoding a frame based on other, “reference,” frames. For example, video signals often exhibit temporal redundancy in which frames near each other in the temporal sequence of frames have at least portions that are match or at least partially match each other.
Multimedia processors, such as video encoders, may encode a frame by partitioning it into blocks or “macroblocks” of, for example, 16×16 pixels. The encoder may further partition each macroblock into subblocks. Each subblock may further comprise additional subblocks. For example, subblocks of a macroblock may include 16×8 and 8×16 subblocks. Subblocks of the 8×16 subblocks may include 8×8 subblocks, which may include 4×4 subblocks, and so forth. As used herein, the term “block” refers to either a macroblock or a subblock.
Encoders take advantage of temporal redundancy between sequential frames using inter-coding motion compensation based algorithms. Motion compensation algorithms identify portions of one or more reference frames that at least partially match a block. The block may be shifted in the frame relative to the matching portion of the reference frame(s). This shift is characterized by one or more motion vector(s). Any differences between the block and partially matching portion of the reference frame(s) may be characterized in terms of one or more residual(s). The encoder may encode a frame as data that comprises one or more of the motion vectors and residuals for a particular partitioning of the frame. A particular partition of blocks for encoding a frame may be selected by approximately minimizing a cost function that, for example, balances encoding size with distortion, or perceived distortion, to the content of the frame resulting from an encoding.
Inter-coding enables more compression efficiency than intra-coding. However, inter-coding can create problems when reference data (e.g., reference frames or reference fields) are lost due to channel errors, and the like. In addition to loss of reference data due to errors, reference data may also be unavailable due to initial acquisition or reacquisition of the video signal at an inter-coded frame. In these cases, decoding of inter-coded data may not be possible or may result in undesired errors and error propagation. These scenarios can result in a loss of synchronization of the video stream.
An independently decodable intra-coded frame is the most common form of frame that enables re/synchronization of the video signal. The MPEG-x and H.26x standards use what is known as a group of pictures (GOP) which comprises an intra-coded frame (also called an I-frame) and temporally predicted P-frames or bi-directionally predicted B frames that reference the I-frame and/or other P and/or B frames within the GOP. Longer GOPs are desirable for the increased compression rates, but shorter GOPs allow for quicker acquisition and re/synchronization. Increasing the number of I-frames will permit quicker acquisition and re/synchronization, but at the expense of lower compression.
There is therefore a need for techniques for video encoding and decoding channel switch frames (CSF) to enable acquisition and re/synchronization of the video stream while preserving compression efficiency.